1. Field of the Invention
This invention relates to a disc type brushless motor, and more particularly to a disc type brushless motor comprising many armature windings and field magnets specially arranged to obtain excellent performance.
2. Description of the Prior Art
Motors are generally classified into AC motors and DC motors and into rotating motors and linear motors. Rotating motors can be divided into core motors and coreless motors or into cup (cylindrical) type motors and disc type motors (flat in the direction of the motor shaft). Rotating motors can also be divided into commutator motors and brushless motors.
Small disc type brushless motors, which have a flat disc-like shape, have recently come to replace cylindrical motors in cassette tape recorders, record players and other audio equipment. Small DC motors of this type are also replacing cylindrical motors in other applications. However, to construct a large disc type brushless motor having a diameter of for example 40 cm for use in special fields such as video equipment, it is necessary to use a very large field magnet, loop-like armature windings, an expensive rotation shaft and expensive bearings. Particularly, it is difficult and very expensive to form a field magnet having a diameter of about 30 cm and comprising 2 m north and south poles, wherein m designates a positive integer, which are alternately arranged at equal intervals. This is partially because the quantity of production of large special disc type brushless motors having a diameter of about 40 cm is limited. Further, to construct a large motor of this type, it is essential to use large loop-like armature windings, which are difficult to form and tend to exhibit insufficient strength because of their large size. Therefore, their strength must be increased by using an expensive plastic molding process, resulting in a high cost as in the case of the above-mentioned field magnet having a diameter of about 30 cm.
As described above, conventional disc type brushless motors having a diameter of for example 40 cm are very expensive. This problem is aggravated by their special applications and by the small quantity of production of such motors compared with that of small motors of this type. Thus a need exists for an inexpensive, efficient, large disc type brushless motors.
Further, motors used in special applications such as video equipment should be disc-shaped and allow special rotational drive at an extremely high efficiency. They should also be highly durable since they are used in a very expensive apparatus. Brushless motors are suitable for this purpose because they can be shaped into a disc and because they have long service life because of the absence of commutators. However, to greatly increase the efficiency of disc type brushless motors, it is necessary to use very many armature windings. In this connection, the conventional motors of this type are disadvantageous as described below with reference to FIGS. 1 to 7.
FIG. 1 is a plan view showing a bow-like armature winding used in a conventional disc type brushless motor, FIG. 2 is a plan view showing a number of the armature windings of the type shown in FIG. 1 arranged for use in a motor, and FIG. 3 is a developed view showing the arrangement of the armature windings of FIG. 2 and a field magnet. In FIG. 1, the bow-like armature winding 2 used in a conventional disc type brushless motor is formed so that the angle (180.degree.) between radial conductor sections A is approximately equal to the pole width (180.degree.) of a two-pole field magnet 1 shown in FIG. 3. The flat disc type brushless motor (not shown) has an interior space in which a rotation shaft is supported approximately at the center of the disc perpendicularly thereto. The field magnet 1 is secured perpendicularly to the rotation shaft. In the interior space of the motor, 13 bow-like armature windings 2 as shown in FIG. 1 are superposed one upon another as shown in FIG. 2 and opposed to the field magnet 1. FIG. 3 shows the development of the field magnet 1 and the armature windings 2 superposed one upon another as described above. It will be understood mainly from FIGS. 2 and 3 that, with the conventional disc type brushless motor, all of conductor sections A, B and B' of the armature windings 2 overlap one upon another, and the thickness thereof becomes extremely large. As a result, the air-gap between the mounts of the field magnet 1 and the armature windings 2 increases, making it impossible to obtain high motor torque and efficiency. Further, in case the armature windings 2 are superposed one upon another in a motor, it is difficult to treat the coil ends. Thus the conventional disc type brushless motor is not suitable for mass production, requires a high production cost, and is difficult to construct in the form of an extremely thin disc.
To solve the above-mentioned problems, it has recently been proposed to increase the number of north and south poles alternately magnetized at equal intervals, decrease the angle between the radial conductor sections of the armature windings 2 to match the increase in the number of magnetic poles, increase the number of the armature windings 2, and position the armature windings 2 at equal intervals so that they do not overlap one upon another. This type of brushless motor can be constructed in a form thinner than that shown in FIGS. 1 to 3, but is not yet completely satisfactory for the reasons described below. Namely, also in this disc type brushless motor, even if the number of the north and south poles of the field magnet is increased, the armature windings naturally overlap one upon another when the number thereof increases.
FIG. 4 is a plan view showing an arrangement of seven armature windings used in a conventional disc type brushless motor, and FIG. 5 is a developed view showing the arrangement of the armature windings of FIG. 4 and a four-pole field magnet. In FIGS. 4 and 5, the armature windings 2 are positioned to minimize overlapping thereof. Generally, it will be possible to position the armature windings 2 so that they do not overlap one upon another if the number of the north and south poles of the field magnet is increased and the angle between the radial conductor sections of each armature winding 2 is reduced. However, in case the field magnet has four poles and seven armature windings 2 are used as shown in FIGS. 4 and 5, it is difficult to completely avoid overlapping of the armature windings 2.
FIG. 6 is a plan view showing a 20-pole field magnet, and FIG. 7 is a plan view showing 13 armature windings positioned at equal circumferential intervals so that they do not overlap one upon another. To position many armature windings 2 at equal intervals so that they do not overlap one upon another, it is necessary for example to form a flat doughnut-like field magnet 1 having 10 north poles and 10 south poles alternately positioned at equal intervals as shown in FIG. 6. In this case, as shown in FIG. 7, each armature winding 2 should be looped in a fan shape so that the angle between the radial conductor sections thereof contributing to generation of torque is approximately identical with the pole width of the field magnet 1, and 13 such armature windings 2 should be positioned at equal intervals so as to eliminate overlapping thereof. With the arrangement shown in FIGS. 6 and 7, if the field magnet 1 has more than 20 poles and the number of the armature windings 2 is further increased, the armature windings naturally overlap one upon another. To eliminate their overlapping, the number of poles of the field magnet 1 must further be increased, and at the same time the angle between the radial conductor sections of each armature winding 2 must be reduced to increase the number thereof. However, increasing the numbers of the field magnet poles and the armature windings in this way detracts from the usefulness of the motor.
In a study of conventional disc type brushless motors of the type described above, it was noted that the disc type motors should not be too long in the direction of the rotation shaft but that, in many motors of this type, the radius may be increased. Therefore, it is advantageous to utilize this permissible condition as much as possible. In the past, however, this permissible condition was not noticed, and studies were directed only to the application of cup type motors and known disc type motors.
In making the present invention, study was further conducted to find disadvantages of the arrangement shown in FIGS. 6 and 7, taking the above-mentioned permissible condition into consideration. The study revealed that the following conditions should be satisfied:
(1) The armature windings 2 should be positioned at equal intervals so that they do not overlap one upon another.
(2) The number of the armature windings 2 should be increased to obtain a disc type motor capable of generating a high torque and exhibiting high efficiency. In addition, the motor should be able to rotate smoothly with little torque ripple.
(3) The conditions (1) and (2) should be satisfied without greatly increasing the number of poles of the field magnet 1.
In connection with these basic conditions (1) to (3), the following should be noted:
(4) In case the disc type brushless motor has a large size, for example a diameter of 40 cm, it is very difficult and expensive to form a flat doughnut-like field magnet 1 having a diameter of 30 cm or more as shown in FIG. 6. This problem is aggravated by the generally small number of large special disc type brushless motors having a diameter of about 40 cm which are produced. For example, the cost of dies for making a field magnet 1 having a diameter of 30 cm as shown in FIG. 6 amounts to about ten million yen, and other expensive components are also needed. Thus a need exists for an inexpensive field magnet 1.
(5) As described above, each armature winding 2 is looped so that the angle between the radial conductor sections thereof contributing to generation of torque is approximately equal to the pole width of the field magnet 1. Therefore, when a field magnet 1 having a diameter of 30 cm is used, the armature winding 2 positioned opposite thereto must be in the form of a large loop. However, a large loop armature winding 2 exhibits small strength and tends to break due to external shocks or the like, particularly when the conductor of the armature winding is not so thick. This problem adversely affects reliability and must be eliminated in an expensive motor of this type. Although this problem can be solved by using self-fusing conductors for making the loop-like armature winding 2 or by looping a conductor and fixing the conductor sections of the loop through plastic molding process, these approaches are very expensive, resulting in very expensive motors.